Implications of the carbon isotope and mineral inclusion record for the formation of diamonds in the mantle underlying a mobile belt: Venetia, South Africa

A total of 199 diamonds from the Venetia kimberlite, South Africa, whose mineral inclusion chemistry had already been measured, were analyzed for their carbon isotopic composition. Silicate inclusions in these diamonds either belong to a peridotitic (P-Type), an eclogitic (E-Type) or transitional, websteritic (W-Type), paragenesis.The carbon isotopic composition of 161 P-Type diamonds ranges from delta C-13 = -2.23 to - 18 parts per thousand vs. PDB. The large number of samples available and the wide range in delta C-13 permitted, for the first time, a detailed analysis of the relationships between P-Type inclusion chemistry and the carbon isotopic composition of the diamond host. The delta C-13 Sampling frequency distribution is multi-modal. Examination of the inclusion chemistry (chromite, olivine, garnet) as a function of the carbon isotope composition mode to which the host belongs, as well as multivariate regression analyses, revealed no correlation between inclusion chemistry and C-13 content. The inclusion compositions in diamonds of low C-13 content are not distinctive. For a given carbon isotopic composition the combination of Ni/Fe and Mg/(Mg + Fe) of olivine inclusions varies systematically along fractionation trends. The composition of the olivine inclusions and the C-13 content of their hosts can be interpreted as reflecting similar petrogenetic processes occurring in several mantle environments into which carbon of variable isotopic composition was introduced. The iron/magnesium distribution between coexisting garnets and olivines permits an estimate of their pressure/temperature equilibration conditions. Diamonds whose inclusions were equilibrated at lower temperatures and pressures tend to have, on average, lower C-13 contents. The compositions of coexisting olivines and chromites suggest oxygen fugacities between 2.9 and 5.8 log units below the quartz-fayalite-magnetite buffer at 50 kbar, and temperatures between 1280 and 1490 degreesC prevailed during diamond formation. Inclusions from diamonds of lower C-13 content do not indicate systematically lower fO(2) values during their formation. The fO(2)/T conditions determined suggest minimal (0.0 to -0.5 parts per thousand) isotope fractionation between a C-H-O vapor phase (carbon dioxide, carbon monoxide, methane, water, hydrogen) and diamond. The large C-13 depletion of some Venetia P-Type diamonds appears to be unrelated to the composition of their inclusions, igneous fractionation trends, oxygen fugacity, and vapor isotope fractionation processes. This conclusion is consistent with evidence deduced from more limited data sets from other kimberlites.